Adds a BUILD_SEED make variable that can be set to a 32-bit integer,
and adds an FPGA read-only register initialized by this variable.
Changing this variable will randomly affect the FPGA build results.
This can be useful when your build fails timing by a small amount,
allowing you to build again and get different results with the same
code.
For example, you could run the following to produce an FPGA build with
different timing results from the default build:
make X310_XG BUILD_SEED=123
Original-commit: 3a3a03486f9221c2e59fa6d0826c23d17730956d
Wade Fife